Beams, trusses, joists and columns are the typical structural members that support the weight or loads of structures, including buildings and bridges. Structural members may be manufactured from a variety of materials, including steel, concrete, and wood, according to the structure design, environment and cost.
Wood is a desirable material for use in many structural members because of its various characteristics, including strength for a given weight, appearance, cyclic load response, and fire resistance. Wood structural members are now typically manufactured from multiple wood segments that are bonded together with their lengths generally aligned, such as in glue-laminated members, solid sawn members and solid sawn web I-beams. These manufactured wood structural members have replaced sawn lumber or timbers because the former has higher design limits resulting from better inspection and manufacturing controls.
Various cutting methods are used to obtain wood segments. One method is known as flat sawing in which a log is sawn along its length to produce the segments. Another method is known as radial sawing in which the segments are obtained by cutting the log into quarters and then cutting each quarter toward the center of the log. Regardless of the method used, each segment has a cross-section with an annual growth ring pattern.
There are generally two types of annual growth ring patterns. The first type is a tangential growth ring pattern in which the annual growth rings intersect the broad surface of the segment at an angle of between zero and thirty degrees. The second type is a radial growth ring pattern in which the annual growth rings intersect the broad surface of the segment at an angle of between thirty and ninety degrees.
After the segments have been cut, they are generally randomly selected for the manufacture of the wood structural member regardless of their annual growth ring pattern. Thus, the wood structural members are composed of randomly placed segments resulting in a random arrangement of annual growth ring patterns. Since segments with a tangential growth ring pattern have different strength characteristics in some applications than segments having a radial growth ring pattern, the structural member may not be capable of withstanding stress produced by an applied load.
One type of stress produced by a load applied to a structural member is shearing stress or shear. Shear is generally defined as the internal force acting along a plane between adjacent parts of a body when two equal forces parallel to the plane act on each part in opposite directions. Shear resists the tendency of one part of the body to slide over the other part. The wood structural member must be capable of bearing the shear stress without excessive strain and particularly without ultimately failing.
Wood structural members are subjected to various types of shear stress which it must resist to prevent excessive strain or failure. Wood structural members most often fail because of horizontal shear stress. Horizontal shear stress acts along a plane perpendicular to the applied load. It is particularly important that the wood structural member be highly resistant to horizontal shear stress. The horizontal shear stress in a wood structural member is highest within about the centermost sixty percent portion of the member. Therefore, it is desirable that the centermost sixty percent portion of the wood structural member be highly resistant to the horizontal shear stress. The horizontal shear stress acting on the structural member outside of its central sixty percent is negligible. Although prior wood structural members are strong and can support a substantial load, such a load over time may cause stress points to develop within the wood structural member and particularly within the centermost segment.
Prior attempts to strengthen wood structural members by the addition of reinforcement panels have been successful. For example, U.S. Pat. No. 5,362,545 describes a glue laminated wood beam with a synthetic reinforcement panel added to the areas of the beam subject to the greatest stress to improve the tensile and compressive strength of the beam. In order to adhere the reinforcement panel to the beam with a non-epoxy adhesive, the surface of the reinforcement panel is treated to cause the synthetic fibers to "hair up." Although this beam does have improved tensile and compressive strength, it requires the additional surface-treated reinforcement panel.
Another example of a reinforced structural member is seen in U.S. Pat. No. 5,498,460 which describes a wood beam with synthetic reinforcements that are positioned in areas of high tensile and compressive stress and adhered to the wood segments. The reinforcements extend approximately three-fifths the length of the beam. The reinforcements are surface-treated to create randomly spaced recesses on their surfaces to facilitate adhesion to the wood segments. The strength of this beam is improved by the addition of the reinforcements.